The ability to accurately characterize the thermal (T) and/or temporal (t) response of a material""s specific volume (Vsp) is critical in materials engineering. This is due in part to a constant influx of new engineered materials to the market place (i.e. thermoplastics, thermosets, metal alloys, polymorphs of existing materials, thin films, nanocomposites, etc.) and the emergence and/or growth of new and exciting fields (i.e. microelectronics, composite manufacturing, etc.). Ultimately, this need is driven by the fact that there are a variety of physical phenomena that result in dimensional change (i.e. crystallization, melting, glass formation, secondary transitions and physical aging).
Although volumetric dilatometers have been in use for decades1,2, at present, no commercially available device exists for the characterization of Vsp(T,t) at ambient pressures that is capable of resolving the transitions alluded to above.
Devices which can be found within the marketplace that are capable of resolving changes of specific volume (Vsp) are as follows:
1. Density gradient columns3,
2. Balance assemblies which allow for density characterization through buoyancy methods4,5,
3. Mercury-in-Glass capillary dilatometers6,
4. Linear dilatometers7,8,
5. Mercury porosimeters9,10,
6. Picnometers9,10 
7. Cylinder-piston type dilatometers, and
8. Pressure-volume-temperature (pvT) devices11,12.
Of these devices, only the latter four (5,6,7 and 8) are able to track volume as a function of both time and temperature and all suffer from limitations which include: 1) a narrow dynamic temperature range, 2) low sensitivity, 3) low accuracy, 4) toxicity concerns and/or 5) pressure requirements.
In recent years, many researchers have looked at supplanting traditional xe2x80x9clinearxe2x80x9d dilatometer sensor technology with optical technologies, yet to date, none have developed an optical technique for use with volumetric dilatometers. In fact, the only attempt toward advancement made in the field of volumetric dilatometry can be referenced in U.S. Pat. No. 5,172,977. This patent employs a capacitive technique for monitoring displacement of a mercury column. While capacitive sensor techniques have recently proven to be highly accurate methods for monitoring displacement in xe2x80x9clinearxe2x80x9d dilatometry13, the applicability of capacitive sensor technology to volumetric dilatometry is questionable14.
FIG. 1 illustrates the sensing principle behind current state-of-the-art for pvT apparatuses, such as the Gnomix device. The limitations associated with the device are 1) a minimum pressure requirement of 10 bar, 2) low sensitivity, and 3) large sample size requirements. These limitations are related to the load cell design, sensor technology and heat source.
13 M. Rotter, H. Muller, E. Gratz, M. Doerr, and M. Loewenhaupt, xe2x80x9cAminiture capacitance dilatometer for thermal expansion and magnetostrictionxe2x80x9d, Review of Scientific Instruments, 69(7), 2142-2746, 1998. 
14 G. J. F. Holman and C. A tenSeldam, xe2x80x9cA critical evaluation of the thermophysical properties of mercuryxe2x80x9d, J. Phys, Chem. Ref Data, 23(5), 807-827, 1994. 
FIG. 2 shows photographs representative of prior art high resolution volumetric dilatometers. The left-hand photo shows a low temperature unit, the center photo shows a high temperature unit, and the right-hand photo shows multiple sample cells which have been constructed of low expansion glass. Note the counter balance systems and the linear voltage differential transducers (LVDT) used to measure displacement in the left and center photos. Both of these features can be shown to limit the ultimate resolution of the device. In addition, note that each glass cell differs in terms of its volume due to the fact that each cell must be manually sealed after loading. This also limits the ultimate sensitivity of the device due to calibration uncertainties associated with the initial cell volume.
FIG. 3 shows the thermal breakers necessary to limit thermal transfer from the heat source to the sensor (LVDT). One breaker is between the lower (invar) portion of the push rod and the upper non-ferrous portion of the push rod. The other breaker is the outer device which houses the LVDT.
FIG. 4 shows the current state-of-the-art configuration for volumetric dilatometers. Shown are the LVDT, the push-rod, the calibrated capillary, the float (aluminum plug) and the heat source.
Generally, there have been four limiting factors associated with volumetric dilatometry. These limitations are derived from: 1) the sensor technology, 2) the cell design, 3) the measurement methodologies, and/or 4) the thermal control strategies employed. FIG. 5 details the limiting factors associated with conventional volumetric dilatometry: the thermal noise of the detector, the error associated with the push rod assembly (most likely attributed to buoyancy issues), and the error associated with changing the counter-balance mass. Note that the counter-balance mass changes with each experiment.
The invention in its preferred embodiment addresses the above and other limitations associated with the prior art. In a first aspect, the invention includes a dilatometer which utilizes a novel fiber optic sensor technology along with associated electro-optic subsystems which utilize a novel signal-processing algorithm. In a second aspect, the invention provides a novel sealed cell design. In a third aspect, the invention provides a novel heat source technology.
The volumetric dilatometer of the invention according to a preferred embodiment provides:
A sensitivity which exceeds 2.5 E-6 cm3,
accuracy which exceeds 2.5 E-5 cm3,
A dynamic temperature range of 300C or greater,
Temperature control to within +/xe2x88x920.005 C, and
A reusable, sealed cell design that allows for ease of operation and reduced environmental concerns associated with traditional mercury-in-glass capillary dilatometers (i.e. mercury toxicity).